Creo que esta es una unión, suma y grupo bastante simple por:
SELECT t.r_id, SUM(t.work) work_sum
FROM tasks t
JOIN info i
ON i.id = t.r_id
AND i.date BETWEEN xxx AND yyy
GROUP BY t.r_id
Si desea un work_sum nulo para tareas sin información en ese rango:
SELECT t.r_id, SUM(t.work) work_sum
FROM tasks t
LEFT JOIN info i
ON i.id = t.r_id
AND i.date BETWEEN xxx AND yyy
GROUP BY t.r_id
Si desea 0 work_sum para tareas sin información en ese rango:
SELECT t.r_id, COALESCE(SUM(t.work),0) work_sum
FROM tasks t
LEFT JOIN info i
ON i.id = t.r_id
AND i.date BETWEEN xxx AND yyy
GROUP BY t.r_id